The invention is in the field of nickel-base super alloys for casting particularly intended for vacuum casting, by directional solidification, of thin wall products of complex shape design for use under extremely demanding service conditions. Particularly is the alloy of this invention intended for use in the casting of turbine blades and vanes for jet engines where the castings will be subjected to extended periods of operation under very high temperatures. For some time equiaxed grain castings were used for such parts. However, these proved unsatisfactory as the operating conditions for the castings became more severe. In response to these problems the gas turbine industry developed the technique of directional solidification (referred to as DS) with significant improvement in high temperature alloy properties and blade performance. However, parts cast by this technique are susceptible to grain boundary cracking.
Designs to increase the power and efficiency of gas turbine engines have required reduction of the wall section of such turbine blade castings to improve blade cooling efficiency. This necessitates the use of special, high strength, cast alloys. Extremely thin wall sections are required to provide rapid thermal transfer because the blades may be operating in a gaseous environment temperature above the melting point of the alloy. In such an environment it is critical to provide sufficiently rapid heat transfer to assure the actual temperature of the blades being kept below that at which they lose structural integrity. As the wall thickness of the blades has been decreased, the strength of the alloys has had to be increased. However, as the strength of the alloy has been increased, the problem of grain boundary cracking during DS casting of the blades has also increased. The response to this has been the development of improved alloys such as those disclosed in U.S. Pat. Nos. 3,526,499 entitled "Nickel Base Alloy Having Improved Stress Rupture Properties" issued Sept. 1, 1970 to R. J. Quigg et al, and 3,677,747 entitled "High Temperature Castable Alloys and Castings" issued July 18, 1972 to C. H. Lund et al and 3,720,509 entitled "Nickel Base Alloy" issued Mar. 13, 1973 to W. P. Danesi et al. Despite these improvements in ultra high strength alloys suitable for directional solidification, grain boundary cracking has continued to be a problem. This invention provides an alloy with a very significant improvement in the elimination of grain boundary cracking.
It has now been found that the super alloy of this invention, due to heretofore undiscovered characteristics of the alloy and of castings made from the alloy, provides a major solution to the problem of lack of tensile ductility and fracture toughness of highly stressed castings utilized under high temperature conditions such as equiaxe, integrally cast, turbine engine wheels. It has been discovered that the changes in the alloy composition over that of related, prior art alloys of the same general type produces a change in carbide morphology, which is a change in shape, as well as in quantity. This change in carbide morphology was unexpected and, until recently, undiscovered.